The Term "polyphenylene ether resin" is well known as defining a class of thermoplastic materials which possess outstanding physical properties, including hydrolytic stability, dimensional stability and excellent dielectric characteristics. Methods of preparation are known in the art and described in the patent literature, e.g., Hay, U.S. Pat. Nos. 3,306,874, and 3,306,875 and Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358, incorporated herein by reference to save unnecessary detail. Other patents which show the preparation of polyphenylene ether resins include Bennett and Cooper, U.S. Pat. Nos. 3,639,656 and 3,838,102, as well as Cooper and Bennett, U.S. Pat. Nos. 3,642,699, and 3,733,299, also incorporated herein by reference.
It is known that when the polyphenylene ether resins are combined with styrene resins, the resulting compositions possess properties which are improved over the corresponding properties of either resin alone. See Cizek, U.S. Pat. No. 3,383,435, the disclosure of which is incorporated herein by reference.
A shortcoming of these thermoplastic materials is their relatively poor resistance to electrical arcing. Such arcing is most conveniently quantized by the related parameter of tracking (often referred to as "arc tracking") resistance. It measures, for example, the ability of the thermoplastic composition to withstand extreme electrical stresses through the rate of formation of a carbonized path or track as two charged electrodes are drawn apart on its surface.
A standard means for measurement of arc/tracking resistance is described in the UL test procedure entitled "High Voltage Arc/Tracking Rate". Desirably, rates for thermoplastic compositions in accordance with such procedure are less than 1 inch/minute. More desirably, a rate of less than 1 inch/5 minutes is obtained.
It is known that the arc/tracking resistance of certain polymeric compositions may be improved through the incorporation of mineral fillers. Even for polyphenylene ether compositions containing in excess of 30% filler by weight, however, desirable arc/tracking rates have not been obtained. As a result, the utility of this important class of compositions has been restricted.